The reproductive status of male rhesus macaques was monitored from birth to senescence using histological and endocrine criteria (testes and serum samples being obtained through the ORPRC Tissue Distribution Program). The testes of neonatal, 1-year-old, and 2-year-old animals were were poorly differentiated and composed mainly of sex cords containing Sertoli cells and primordial germ cells. The 3-year-old animals showed variable degrees of testicular differentiation representing progressive stages of pubertal development from minimal differentiation to complete spermatogenesis. All animals 4 years of age and older were spermatogenically active. Throughout development, the diameter of the seminiferous tubules was positively correlated with the volume of the testis (r=0.984) and in terms of these parameters the spermatogenic capacity of the testis increased progressively from puberty into the third decade of life. In the oldest animals studied (26-27 years of age), the tubule and testis dimensions significantly exceeded those of both pubertal animals and young adults. Both serum LH and testosterone concentrations were elevated during the neonatal period (<1 year) and fell to a nadir during infancy (1-2 years). In older animals (3 years and older), the increase in spermatogenic capacity of the testis was associated with a progressive increase in serum testosterone concentrations and variable elevations in serum LH concentrations. These data emphasize the extreme extension of reproductive quiescence that characterizes infancy in primates. On the other hand, they reveal that once spermatogenic and endocrine function is established it can be sustained for 25 or more years, well into somatic senescence.